Various tire constructions have been suggested for pneumatic runflat tires, that is, tires capable of being driven on in the uninflated condition. One approach described in U.S. Pat. No. 4,111,249 entitled the "Banded Tire" was to provide a hoop or annular band directly under and approximately as wide as the tread. The hoop in combination with the rest of the tire structure could support the vehicle weight in the uninflated condition. This banded tire actually tensioned the ply cords even in the uninflated condition.
Another approach taken has been to simply strengthen the sidewalls by increasing the cross-sectional thickness thereof. These tires when operated in the uninflated condition place the sidewall in compression. Due to the large amounts of rubber required to stiffen the sidewall members, heat build-up is a major factor in tire failure. This is especially true when the tire is operated for prolonged periods at high speeds in the uninflated condition. Pirelli discloses such a tire in European Pat. Pub. No. 0-475-258A1.
A Goodyear patent having some of the same inventors of the present invention disclosed the first commercially accepted runflat pneumatic radial ply tire, the Eagle GSC-EMT tire. The tire was accepted as an equipment option for the 1994 Corvette automobile. U.S. Pat. No. 5,368,082 teaches the employment of special sidewall inserts to improve stiffness. Approximately six additional pounds of weight per tire was required to support an 800 lb load in this uninflated tire. These runflat tires had a very low aspect ratio. This earlier invention although superior to prior attempts still imposed a weight penalty per tire that could be offset by the elimination of a spare tire and the tire jack. This weight penalty was even more problematic when the engineers attempted to build higher aspect ratio tires for the large luxury touring sedans. The required supported weight for an uninflated luxury car tire approximates 1400 lbs. load. These taller sidewalled tires having aspect ratios in the 55% to 65% range or greater means that the working loads were several times that of the earlier 40% aspect ratio runflat Corvette type tires. Such loads meant that the sidewalls and overall tire had to be stiffened to the point of compromising ride. Luxury vehicle owners simply will not sacrifice ride quality for runflat capability. The engineering requirements have been to provide a runflat tire with no loss in ride or performance. In the very stiff suspension performance type vehicle the ability to provide such a tire was comparatively easy when compared to luxury sedans with a softer ride characteristic. Light truck and sport utility vehicles, although not as sensitive to ride performance, provide a runflat tire market that ranges from accepting a stiffer ride to demanding the softer luxury type ride.
An equally important design consideration in the development of a runflat tire is insuring that the uninflated tire remains seated on the rim. Solutions have been developed employing bead restraining devices as well as special rims to accomplish this requirement such as Bridgestone Expedia S-01 Runflat A/M Tire. Alternatively, the Eagle GSC-EMT tire employed a new bead configuration enabling the tire to function on standard rims without requiring additional bead restraining devices.
Two U.S. Pat. Nos. 5,427,166 and 5,511,599 of Walter L Willard, Jr., show Michelin tires that disclose the addition of a third ply and an addition of a third insert in the sidewall to further increase the runflat performance of the tire over the original U.S. Pat. No. 5,368,082 Oare et al. patents. These patents discuss some of the load relations that occur in the uninflated condition of the tire and they demonstrate that the Oare et al. concept can be applied to addition numbers of plies and inserts.
A latter runflat tire attempt is taught in U.S. Pat. No. 5,685,927 which provides a higher aspect ratio tire with the employment of a load supporting bead core placed directly under the tread belt package of the tire. Again, most of the inventors of that concept were part of the original design team of the original Corvette EM tire. Although very promising in load support and ride that approach showed somewhat higher rolling resistance in the normally inflated conditions.
A further latter U.S. Pat. No. 5,535,800 discloses the use of elastomeric covered composite ribs that in combination with a radial ply can provide excellent runflat capability in a wide range of tire applications.
An object of the present invention was to provide a tire having limited runflat mileage without appreciably increasing the tire's weight, rolling resistance or diminishing the overall ride performance.
A second objective was to apply the inventive concept to a variety of alternative carcass constructions.